As geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Techniques of this type, known generally as optical metrology, operate by focusing an optical beam on a sample and then analyzing the reflected energy. Spectroscopic ellipsometer (SE) is a widely used form of optical metrology that is particularly useful for analyzing multilayer film stacks formed on semiconductor wafers. Briefly, ellipsometry measures polarization properties of a sample, commonly termed “ellipsometric parameters.” These ellipsometric parameters are defined as the ratio of magnitudes and difference of phase for light of two orthogonal polarization states, commonly referred to as s-polarized and p-polarized light. By measuring these ellipsometric parameters over a broadband of spectrum spreading from deep ultraviolet (DUV) to near infrared (NIR), SE can determine thicknesses and CDs of multiplayer film stacks.
As the geometries used in semiconductors continue to decrease, optical metrology tools are forced to analyze smaller and smaller structures. For most optical metrology systems, this means using smaller measurement spots (the area within a subject that the detected light originates from during measurement). At the same time, it is not always practical to reduce measurement size, particularly for ellipsometers. This is partially because ellipsometers, unlike reflectometers, are typically configured to operate at non-normal angles of incidence. The non-normal angle of incidence increases sensitivity to thin-film properties. At the same time, non-normal incidence elongates the measurement spot by a factor equal to 1/cos(θ) where θ is the angle of incidence. For an incident angle of seventy-degrees, for example, this elongation means that the measurement spot is spread to nearly three times its normal length.
Chromatic aberration is a second obstacle that often limits reductions in measurement spot sizes for ellipsometers. Chromatic aberration results when an optical system transports light in a wavelength dependent fashion. In spectral ellipsometers, the probe beam includes a range of wavelengths and chromatic aberration tends to create different measurement spot sizes for the different probe beam wavelengths. This is particularly true for spectral ellipsometers that use diffractive optical elements. The overall result is that the minimum size of the measurement spot is influenced by the range of wavelengths included in the probe beam and the amount of chromatic aberration present of the spectral ellipsometer.
As shown in FIG. 1, one approach for reducing measurement spot sizes in ellipsometers is to use normal incidence in combination with a high numerical aperture objective. The use of the high numerical aperture objective increases the accuracy with which the measurement spot may be imaged. The high numerical objective also creates a spread of angles of incidence all converging on a relatively small illumination spot. The angles of incidence can range up to 70 degrees where a numerical aperture of 0.95 is used. For a more typical case, a numerical aperture of 0.9 is used and the angles of incidence are as high as 64 degrees. In either case, the multiple angle of incidence approach provides an enhanced ability to deduce thin film properties while still maintaining a small measurement spot size.
Systems of the type shown in FIG. 1 are generally referred to as Beam Profile Ellipsometers (BPEs) and are described in more detail in U.S. Pat. Nos. 5,596,411, 5,877,859, 4,999,014, 5,042,951, 5,181,080, 5,412,473, 5,596,406, 6,304,326, and 6,429,943 (the disclosure of each of these documents is incorporated by reference).
In practice, there are a number of obstacles that must be overcome to optimize BPE performance for small measurements spot sizes. One of these is separation of the ellipsometric parameters related to the sample from those related to the focusing optical elements. This problem arises because the optical components that direct the probe beam within an ellipsometer have their own ellipsometric parameters. These parameters must be distinguished from the ellipsometric parameters of the sample before accurate measurements may be made. For BPE systems, separation is difficult to achieve because the objective lens is used for both illumination and imaging and must, as a result, be placed in close proximity to the sample. The nearness in proximity means that the ellipsometric parameters related to the sample cannot be separated from those contributed by the objective.
An obvious approach is to attempt to reduce or minimize the contribution of the objective to the measured ellipsometric parameters. In practice, this turns out to be a difficult goal to achieve. One reason for this is the size and complexity of the high numerical aperture objective. Mounting this lens in a stress free fashion is not generally possible. Temperature fluctuations and air turbulence (often caused by wafer movements) induce additional stress on the objective. Stress causes the crystalline lattice of the objective to deform making the objective birefringent (i.e., the objective exhibits different refractive indices for s-polarized and p-polarized light). The birefringence changes as components expand, contract or move in response to heat, turbulence and other stresses.
One method for reducing birefringence-induced effects is to align the polarization direction of the incident beam parallel to one of the axes (either the fast or the slow axis) of the birefringent component. This scheme, however, cannot be easily implemented to solve the objective effects. There are two issues here: 1) the orientation of the stress changes as the environment perturbations are random in nature, and 2) even given the orientation of the stress, its effects can only be eliminated from the incident beam. The beam reflected from the sample is usually elliptically polarized, and thus will be inevitably affected by the objective no matter which orientation of the stress.
Another method for reducing birefringence-induced effects is to calibrate using a set of standard wafers with known structures. In practice, calibration with standard wafers only works if the wafers are first characterized using a tool that has a higher accuracy that the ellipsometer being calibrated. This characterization must be repeated periodically as matter accumulates on the surface of the standard wafers. The process of characterization is expensive and time consuming (especially when repeated) and, in practice, may not be convenient or cost effective. Calibration also creates many cycles of wafer movements as sample wafers. Typically, this means that the sample wafers are first removed to standard wafers to be loaded. One or more standard wafers are then loaded and unloaded in succession followed by reloading of the sample wafer. This not only reduces throughput, but also creates air turbulence during measurements. Because the objective in BPE systems is highly sensitive to environment perturbations, these repeated wafer movements often lead to calibration errors.
Based on the preceding description, it is clear that there is a need for methods that increase ellipsometer accuracy by separating the ellipsometric parameters related to the sample from those related to the focusing optical elements. This is increasingly important as design rules for semiconductor wafers continue to shrink and is increasingly important for measuring multiple films within multilayer film stacks.